A method of drying granular plastics is described in U.S. Pat. No. 4,509,272, according to which hot dry air flowing through a bed of the granulate removes the moisture from the granulate. The moist air leaving the granulate drying operation is then passed through an adsorbent material, which is usually a molecular sieve material or a mixture of a molecular sieve material with silica gel, whereby the moisture which entered the air stream from the granulate bed is removed from the air stream by adsorption. When the adsorbent material becomes saturated with moisture, it is subjected to regeneration, wherein hot regeneration gas is passed through the adsorbent material, causing the water contained in the adsorbent material to vaporize and be transported away in the regeneration offgas (regeneration exhaust air). After completion of the regeneration, the thereby dried adsorbent material is reused for adsorbing moisture by passing moist air from the granulate vessel through the adsorbent material.
This means of drying moist exhaust air from granulate drying is very effective, but is relatively energy intensive. A large amount of energy is lost from venting the hot regeneration offgas at the end of the regeneration cycle. Attempts have been made to pass the regeneration offgas from the adsorbent material through a heat exchanger, to warm the regeneration gas drawn in by suction from the exterior, prior to feeding the gas through the adsorbent material. A problem presented is that, particularly at the beginning of the regeneration cycle, the hot regeneration offgas from the adsorbent material undergoing regeneration carries substantial water vapor, which is deposited in the heat exchanger. This leads to premature corrosion of the heat exchanger. Further, the heat exchanger adds to the capital and maintenance costs of the apparatus.